1. Field of the Invention
The present invention relates to equalizer devices, and more particularly, to an equalizer device for performing equalization.
2. Description of the Related Art
In transmission systems, signal processing called equalization is performed. Equalization refers to a process of reducing distortion caused on transmission paths, and usually, distorted frequency characteristics are made up for by an equalizer at the receiving side.
For example, in mobile communications, radio signal undergoes multipath (phenomenon wherein a signal wave is propagated through multiple paths while being reflected by mountains, buildings and the like) before arriving at the receiving side. Consequently, the times of arrival of individual waves vary depending on the propagation path length, causing delay distortion of the transmitted waveform. Also, since the signal wave is repeatedly reflected and scattered before arriving at the receiving side, amplitude distortion occurs.
If delay distortion or amplitude distortion occurs, intersymbol interference is caused wherein the transmitted pulse overlaps with its adjacent pulses, making it impossible for the receiving side to accurately distinguish the transmitted pulse. It is therefore necessary that equalization should be performed following up change in the transmission path characteristics, in order to remove the intersymbol interference and thereby compensate for degradation in the transmission quality.
An equalizer used to perform such equalization is an adaptive equalizing filter in which a desired signal is looked up and weighting factors (tap coefficients) of the filter are adaptively set so that the phase and amplitude of the filter output may approach those of the desired signal. Generally, FIR (Finite Impulse Response) filters are widely used (the desired signal is called training signal).
The equalizer is used in W-CDMA (Wideband-Code Division Multiple Access), which is a major mobile communication scheme, in the manner described below. At the transmitting side, a pilot signal (CPICH: Common Pilot Channel) and a data signal are subjected to CDM (Code Division Multiplex) and the resulting multiplexed signal is transmitted. At the receiving side, CPICH is input, as the training signal, to the equalizer to carry out channel estimation etc.
In mobile communications, variation in the strength or phase of the received signal, namely, fading, occurs due to various factors present in the course of radio wave propagation such as multipath. Fading is a cause of increase in the information transmission error rate.
Accordingly, in W-CDMA, the transmitter performs code division multiplexing on the data signal and CPICH containing symbols whereby the receiver identifies the modulation phase of the transmitted data. The receiver refers to the phase/amplitude of the received CPICH and corrects the phase/amplitude of the data signal for channel estimation. By performing synchronous detection on the basis of the result of the channel estimation, it is possible to demodulate data with accuracy even in a fading environment.
A conventional equalizer adapted to equalize a code division multiplexed signal consisting of pilot and data signals is proposed, for example, in PCT-based Japanese Patent Publication No. 2004-519959 (paragraph nos. [0013] to [0026], FIG. 1).
There has also been proposed an equalizer in which an NLMS (Normalized Least Mean Square) adaptive algorithm is applied to CPICH as the training signal (e.g., Moritz Harteneck, Carlo Luschi, “Practical Implementation Aspects of MMSE Equalisation in a 3GPP HSDPA Terminal”, VTC2004-Spring, pp. 445-449).
FIG. 13 illustrates a schematic configuration of an equalizer. The equalizer 50 comprises a tap coefficient variable filter 51, a differential calculator 52, and a tap coefficient corrector 53. The tap coefficient variable filter 51 corrects tap coefficients thereof in accordance with a control signal output from the tap coefficient corrector 53.
The differential calculator 52 obtains a differential value between a filter output signal y(n) (n represents time) output from the tap coefficient variable filter 51 and a training signal d(n), and sends the differential value, as an error signal e(n), to the tap coefficient corrector 53. The tap coefficient corrector 53 performs an algorithm operation so as to minimize the error signal e(n), and outputs the control signal for correcting the tap coefficients of the tap coefficient variable filter 51.
The error signal e(n) is given by Equation (1) below.e(n)=d(n)−y(n)  (1)
In adaptive equalizing filters, the tap coefficients are corrected so as to minimize the mean square error, which is a mean value (expected value) of squares of e(n), and not to minimize e(n) itself (since the received signal is an irregular signal containing, besides the original information, fading variation and noise, it is appropriate to use statistically processed e(n) where e(n) is regarded as an evaluation function).
Adaptive algorithm is an algorithm for successively updating the tap coefficients so that the mean square error of the error signal e(n) may be minimized, and LMS (Least Mean Square) algorithm is widely known as a typical adaptive algorithm. The LMS algorithm is used in various fields because of its stability and also because the number of computations required is small (and therefore, the circuitry can be downscaled).
In the case of correcting the tap coefficients with the use of the LMS algorithm, the filter output signal y(n) is controlled so as to approach the training signal d(n), as seen from Equation (1).
In conventional equalizers provided in receivers for performing W-CDMA wireless communication, the CPICH is used as the training signal and the tap coefficients are updated by performing an LMS algorithm operation so that the equalized filter output signal may approach the CPICH.
However, the filter output signal is derived in the W-CDMA receiver by equalizing the received signal which is a code division multiplexed signal consisting of the CPICH, as the pilot signal, and the data signal. It is therefore not desirable that the CPICH alone be used as the training signal, and the error signal obtained using only the CPICH is not accurate. For this reason, the conventional equalizers are associated with the problem that the tap coefficient correction (updating) accuracy is low.
Specifically, according to the LMS algorithm, an error between an ideal value of equalized output and an actual equalized output is obtained and the equalizer is controlled so as to reduce the error. In the conventional equalizers, however, the actual equalized output contains a sequence of the CPICH as well as a sequence of the data signal, but the training signal as the ideal equalized output value contains only the sequence of the CPICH. Thus, a differential between the signals with low correlativity is obtained for the equalization, which leads to lowering in the reception performance of the W-CDMA receiver.
Also in the conventional techniques disclosed in PCT-based Japanese Patent Publication No. 2004-519959 and “Practical Implementation Aspects of MMSE Equalisation in a 3GPP HSDPA Terminal”, a known signal alone is used as the training signal to obtain an error between signals; hence it cannot be said that optimum adaptive equalization can be performed by these techniques.